In certain applications, pieces are manufactured from steel sheet which has very high mechanical strength. This type of combination is particularly desirable in the automobile industry, where attempts are being made to significantly reduce the weight of the vehicles. This weight reduction can be achieved with the use of steel parts with very high mechanical characteristics and a martensitic microstructure. Anti-intrusion and structural parts, as well as other parts that contribute to the safety of automotive vehicles such as: bumpers, door or center pillar reinforcements and wheel arms, for example, require such characteristics. The thickness of these parts is preferably less than 3 millimeters.
Sheets that have even greater mechanical strength are desired. The ability to increase the mechanical strength of a steel with a martensitic structure by means of an addition of carbon is well known. However, this higher carbon content reduces the weldability of the sheets or of the parts fabricated from these sheets and increases the risk of cracking linked to the presence of hydrogen.